Novolac polymer planarization films with high temperature stability

ABSTRACT

A process for forming a planarization film on a substrate that does not smoke or fume on heating includes applying a polymeric solution including a novolac resin having a weight average molecular weight between about 1000 and 3000 amu, which has been fractionated to remove molecules with molecular weight below about 350 amu, a surfactant selected from a group consisting of a non-fluorinated hydrocarbon, a fluorinated hydrocarbon and combinations thereof, and an optional organic solvent to a substrate, followed by heating the substrate.

RELATED APPLICATION

This application is a division of U.S. application Ser. No. 09/205,006,filed Dec. 2, 1998.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to novolac polymer planarization filmsfor microelectronic devices, such as integrated circuits, and morespecifically to planarization films with high temperature stability.

2. Description of the Related Art

Novolac polymers have been used extensively in the manufacture ofintegrated circuits and other semiconductor and microelectronic devices.In particular, photoresists used for microlithographic patterning ofsemiconductor structures often contain anovolac component along with aphotosensitive agent. See, for example, U.S. Pat. No. 5,601,961 toNakayama et al.

In addition, novolac polymers are also components of planarizing filmsused in the fabrication of microelectronic devices to provide arelatively flat surface. See, for example, U.S. Pat. No. 5,276,126 andreferences therein. As the characteristic feature size on such devicesbecomes smaller, planarizing films are increasingly important in thedevice fabrication process. Low weight average molecular weight novolacpolymers, i.e. those ranging between about 200 and about 2300 atomicmass units (amu) have been found to be useful in forming planarizingfilms because they tend to flow more readily than polymers having highermolecular weights.

In a typical process of forming a planarization film, a solutioncontaining a novolac polymer is formulated with a surfactant. Thesurfactant-containing polymer solution is applied to a substrate byconventional spinning techniques. The polymer solution-coated substrateis heated to evaporate any residual solvent present in the film materialand to reduce the viscosity of the film. The reduced viscosity causesthe material to flow and enhances leveling of the film on the substrate.One difficulty in using these novolac polymer formulations to formplanarizing films is that fuming may be observed on heating. Thermallyvolatilized material is detrimental in that it may form particles thatcan lead to defects in the manufactured devices and may clog vacuumlines.

It would be desirable to provide a process of forming a planarizing filmfrom a novolac polymer material that retains the excellent planarizationof previous materials but does not fume or smoke on heating.

SUMMARY OF THE INVENTION

In accordance with this invention, a process of forming a planarizingfilm on a substrate is provided, the process including first applying tothe surface of the substrate a solution including a novolac resin havinga weight average molecular weight between about 1000 and 3000 amu andwherein the novolac resin is fractionated to remove the molecules withmolecular weight below about 350 amu and a surfactant selected from agroup consisting of a non-fluorinated hydrocarbon, a fluorinatedhydrocarbon and combinations thereof. The process additionally includesheating the solution-covered substrate to form a planarized film.

According to another aspect of the present invention, a substrate havinga planarized film applied thereon is provided, the film comprising anovolac resin having a molecular weight between about 1000 and 3000 amuand wherein the novolac resin is fractionated to remove the moleculeswith molecular weight below about 350 amu and a surfactant selected fromthe group consisting of a non-fluorinated hydrocarbon, a fluorinatedhydrocarbon and combinations thereof.

In yet another embodiment of the invention, there is provided acomposition for use in the formation of planarizing films on substrates,the composition comprising the fractionated novolac resin as describedabove, a surfactant selected from the group consisting of anon-fluorinated hydrocarbon, a fluorinated hydrocarbon and combinationsthereof, and an optional organic solvent. The novolac resins used in thecomposition according to the present invention are fractionated byextraction techniques such as column extraction, liquid-liquidextraction, or supercritical fluid extraction to remove the fractionwith molecular weight below about 350 amu.

Using the composition according to the present invention in formingplanarizing films, no fuming or smoking is observed during the processof heating a coated substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process of forming a planarizing film according to this inventionuses a composition including a novolac resin that has been fractionatedto remove low molecular weight components and a surfactant. Novolacpolymers and surfactants that can be used in this invention aredescribed in detail in U.S. application Ser. No. 08/271,291, entitled“Novolac Polymer Planarization Films for Microelectronic Structures”,(denoted the '291 application), now U.S. Pat. No. 5,595,547 which iscommonly assigned with the present application and is incorporatedherein by reference.

As described above, novolac polymers with low molecular weights areespecially useful in forming planarizing films because they tend to flowmore readily than polymers having higher molecular weights. Polymermolecular weight, as used here, refers to weight average molecularweight, as determined, for example, by gel permeation chromatography,calibrated against polystyrene. According to the present invention, ithas been determined that the lowest molecular weight fraction, that isthose novolac molecules with molecular weight less than about 350 amuare thermally volatilized when the formulation containing novolacpolymers is heated in forming planarizing films. Thus removal of thelowest molecular weight fraction overcomes the problem of fuming orsmoking on heating a coated substrate.

Novolac polymers are commercially available or may be derived fromreacting phenols or derivatives therefrom, such as ortho-, meta- andpara cresol, with formaldehyde or with other aldehyde compounds. Thelowest molecular weight fraction can be substantially removed byextraction techniques such as toluene extraction, column extraction,liquid-liquid extraction, and supercritical fluid extraction.

In the toluene extraction process, solid novolac resin is combined withtoluene and heated to between approximately 75 and 80° C. The toluene isdecanted and a second lot of toluene is added; the mixture is heated;and the toluene is decanted. The remaining solid is a novolac polymerwith the lowest molecular weight fraction reduced. An alternativeextraction process, column extraction, is performed on a mixture ofpolymer resin ground to a fine powder and dry silica gel, installed in aglass column. The mixture in the column is eluted with a first solventmixture, for example, an ethyl acetate and hexane mixture, until a largevolume of elution solvent is obtained, capturing the low molecularweight components. The column is then eluted with a second solvent, forexample, methanol, from which the novolac polymer with the lowestmolecular weight fraction removed is recovered.

In the liquid-liquid extraction process, the novolac polymer is combinedwith two solvents with different polarity, for example, ethyl acetateand hexane, and mixed with a sonicator. The contents separate into twophases; the polymer with the lowest molecular weight fraction removed isrecovered from the bottom layer. A supercritical fluid extractiontechnique can also be used. In this process, flows of a polar solvent,such as ethyl acetate, ethanol, or methanol, and CO₂ are passed over anovolac polymer sample in an extractor vessel, heated to temperatures inthe range between about 60 and 65° C. and pressurized to about 200 to300 bar. The remaining novolac polymer in the vessel is depleted of thelow molecular weight component.

As reported in detail in the appended examples, extraction using theabove techniques results in a novolac polymer with increased molecularweight and narrowed polydispersity. Polydispersity is defined as theratio of the weight average molecular weight to the number averagemolecular weight. For example, extraction of the phenolic novolacdenoted SD-333A, provided by Borden Chemical, Inc. increases themolecular weight from about 900 to between about 1300 and 1800, anddecreases polydispersity from over 1.5 to less than 1.4, depending onthe extraction method. The molecular weight distribution of moleculesthat make up the novolac resin may be determined using gel permeationchromatography (GPC). High performance liquid chromatography (HPLC) isused to determine the removal efficiency of the low molecular weightmaterial that causes fuming. Fractionation reduces the contribution inHPLC area per cent of molecules with molecular weight less than about200 from over 20% to less than about 4% of the total molecular weightdistribution and reduces the contribution of molecules with molecularweight less than about 350 from over 30% to less than about 10%.Furthermore, fractionation improves thermal stability of the novolacpolymers as evidenced by increase in the glass transition temperatureand decrease in the weight loss on heating.

Thus, the novolac polymers used in this invention are specified by theirweight average molecular weight and by the fraction of molecules withmolecular weight less than about 350, i.e. the lowest molecular weightfraction, remaining after fractionation. Novolac polymers with molecularweight between about 900 and about 2500, and preferably between about1200 and 2300, and with the lowest fraction less than about 22%, and,preferably, less than about 15% are advantageously used.

The fractionated novolac polymer can be combined with a surfactant informulating a coating solution for forming planarizing films. Asdescribed in the '291 application, surfactants suitable for thisinvention include non-fluorinated and fluorinated hydrocarbons andmixtures thereof. Suitable non-fluorinated hydrocarbon surfactants maybe comprised of alkylated derivatives of organic acids and estersthereof having from about 5 to about 50 carbons, preferably from about10 to about 30 carbons and combinations thereof. Suitable fluorinatedhydrocarbon surfactants may be comprised of alkylated derivatives oforganic acids and esters thereof having from about 5 to about 50carbons, preferably from about 10 to about 30 carbons, and at least onecarbon-fluorine bond, and combinations thereof. More specifically,particular fluorinated hydrocarbon surfactants include fluoroaliphaticoxyethylene adducts, fluorinated alkyl alkoxylates and sulfonamidescontaining from about 50 to about 20 carbon atoms, fluoroaliphaticpolymeric esters derived from monomers comprised of partiallyfluorinated hydrocarbon chains containing from about 50 to about 20carbon atoms with terminal ester groups attached thereto,fluoroaliphatic copolymers derived from monomers comprised of partiallyfluorinated hydrocarbon chains containing from about 5 to about 20carbon atoms with terminal functional groups selected from esters andacids attached thereto, and combinations thereof. These fluorinatedsurfactants are commercially available from 3M.

An organic solvent may optionally be included as a third component ofthe coating solution. Solvents suitable for this invention includealiphatic and aromatic hydrocarbons, alcohols, ketones, ester, ethers,ether alcohols, ether esters, alcohol esters, ketone esters, ketoneethers, ketone alcohols, amides, nitriles, and combinations thereof.More specifically, particular solvents include ethyl lactate, ethylacetate, propyl acetate, butyl acetate, and combinations thereof.

The polymeric solution preferably contains from about 1 to about 90percent, more preferably between from about 10 to about 50 percent, andmost preferably from about 20 to about 40 percent, based upon the totalweight of the solution, of the novolac polymer, and preferably fromabout 0.01 to about 5 percent, more preferably from about 0.1 to about 1percent, and most preferably from about 0.3 to about 0.7 percent ofsurfactant. The optional solvent may be present in an amount rangingbetween about 10 to about 90 percent, preferably between about 50 toabout 90 percent, and most preferably between about 60 to about 85percent.

The polymeric solution may be applied to the substrate by anyconventional means, such as spin-coating. Preferably, the solution iscentrally applied to the substrate, which is then spun at speeds rangingbetween about 500 and about 6000 rpm, preferably between about 1500 andabout 4000 rpm, for about 5 to about 60 seconds, preferably about 10 toabout 30 seconds. Optionally, an additional, short, lower speed spin,between about 400 and 600 rpm for about 1 to about 5 seconds is used tospread the solution immediately after application.

Typically, the compositions of this invention are applied onto wafersubstrates, such as silicon wafers which have a circuit pattern on theirsurface, to be processed into integrated circuits or othermicroelectronic devices.

The coated substrate is then heated by any conventional means.Preferably, the substrate is heated by placing it on a hot plate to heatthe wafer from below. Typically, this is done commercially via aconventional integrated spin-coater/hot plate system. The coatedsubstrate is typically heated for about 0.5 minutes to about 5 minutesat temperatures ranging between about 50° C. and about 300° C.,preferably between about 100° C. and 200° C. Alternatively, multiple hotplates, i.e. between about 2 and about 5 hot plates, may be used, withthe same time and temperature ranges applying, and where the temperatureof each subsequent hot plate is higher than the temperature of theprevious one.

As illustrated in the following examples, no fumes are observed whenformulations containing fractionated novolac polymers, according to thepresent invention, are coated on substrates and heated as describedabove to form a planarized film.

EXAMPLES

Preparation of fractionated novolac polymers by toluene extraction,liquid-liquid extraction, column extraction, and supercritical fluidextraction is given in Examples 1-4, respectively. The phenolic novolacpolymer, SD-333A, provided by Borden Chemical, Inc. was used in theseexamples. Characterization methods and properties of the fractionatedpolymers obtained in Examples 1-4 are given in Examples 5 and 6. Theoriginal phenolic novolac polymer and the fractionated polymers areformulated with a surfactant and solvent to form coating solutions.Formulation and performance of the coating solutions are reported inExample 7.

Example 1

Solid chunks of novolac polymer (206.5 g) were combined with 8 liters oftoluene in a 12 liter round bottomed flask and heated to 75-80° C. withstirring for 75 minutes. The solid melted above 55° C. The toluene wasdecanted and a second lot of 8 liters of toluene added, heated withstirring for 75 minutes, and decanted. The solid remaining in the flaskwas dissolved in methanol, roto-evaporated until a fluffy pink solid wasobtained and dried overnight in a vacuum oven at 45° C. (88.5 g).

Example 2

Novolac polymer (182.34 g) was placed in a 12 liter round bottom flask.The solvent mixture 30% ethyl acetate/70% hexane, by volume, B & J brandethyl acetate, Fisher Optima grade hexane, (10 liters) was added to theflask and the contents mixed with stirring for 4 hours. After thoroughmixing, the contents separated into two phases, a dark viscous bottomlayer and a cloudy white top layer. The bottom layer, containing thefractionated novolac polymer, was separated and the solvent removed byroto-evaporation. The yield was 49.6 g (27.2%).

Example 3

Solid novolac polymer ground to a fine powder (300 g) was mixed with1200 g of dry silica gel. A 2 inch layer of clean silica gel, slurriedwith 30% ethyl acetate/70% hexane, was installed in a large glass column(3 ⅞ inch×48 inches). The polymer/silica gel mix slurried with the samesolvent was installed over the silica gel. The column was eluted with30% ethyl acetate/70% hexane until 15 liters of elution solvent wascollected. The column was next eluted with methanol and three 4 literfractions were collected. The first two fractions were roto-evaporateduntil a solid was obtained: first fraction (153.4 g), second fraction(7.1 g) for a total yield of 160.5 g.

Example 4

Novolac polymer (19.3 g) was placed in a 50 cc sample cartridge andinserted in a 30 cc extractor vessel of a Marc Sims Dense Gas ManagementSystem, “supercritical fluid extraction apparatus”, fitted with anAlltech model 426 standard HPLC pump for solvent addition. The extractorvessel was purged with CO₂ (Air Products, SFC grade) for 10 minutes at aflow rate of about 2 g/min and heating started. The flow was thenstopped and heating continued until the operating temperature of 60-61°C. was obtained. During heating the HPLC pump was primed with ethylacetate (B&J brand). Pressure in the extractor vessel was 50-250 bar.

Extraction was performed in two stages. The first stage was started withCO₂ and ethyl acetate flows of 9.2-9.4 g/min and 1.0 ml/min,respectively at a pressure of 250 bar. After 3597 g of CO₂ passedthrough the extractor, the temperature was increased to 62° C. After anadditional 4023 g of CO₂ passed through the extractor, the temperaturewas increased to 64-65° C., at which temperature, 4500 g of CO₂ werepassed through the reactor. For the second stage, flows of 9.6-9.8 g/minand 3.0 ml/min of CO₂ and ethyl acetate, respectively, were maintaineduntil 2057 g of CO₂ had passed through the extractor. The vesseltemperature was maintained at 62-63° C. throughout the second stage. Theyield of fractionated novolac polymer was 11.29 g.

Example 5

Weight average molecular weight (M_(w)) and number average molecularweight (M_(n)) were determined by gel permeation chromatography (GPC)with respect to polystyrene. Glass transition temperature was determinedby differential scanning calorimetry (DSC). The DSC measurementprocedure included a 1 minute preheat at 250° C. to remove any residualsolvent, followed by a temperature scan from 25° C. to 200° C. at a rateof 10° C./minute. Thermal weight loss was determined by thermalgravimetric analysis (TGA), in which sample weight was recorded as thetemperature was raised from 30° C. to 300° C. at a rate of 10°C./minute. Final weight differential is recorded below in Table 1.

TABLE 1 Characterization of Resin Material Material M_(w) M_(n)Polydispersity T_(g)(° C.) % Wt Loss Unfractionated  936  606 1.544 36.627.2 Example 1 1436 1055 1.361 72.4 15.2 Example 2 1730 1246 1.388 76.413.9 Example 3 1364 1072 1.273 73.8 13.4 Example 4 1301  916 1.419 64.913.0

Example 6

Results of High Performance Liquid Chromatography (HPLC) analysis,obtained with a Hewlett-Packard HPLC Model 1100 are reported below inTable 2. Area percent for peaks corresponding to specific molecularweights are given. Equal detector response for all molecular weights isassumed.

TABLE 2 Characterization of Resin Material - HPLC Data Material M_(w) ≈200 (area %) M_(w) ≈ 306 (area %) Unfractionated 22.47  9.11 Example 13.05 4.67 Example 2 3.94 3.22 Example 3 0.08 4.13 Example 4 1.32 6.50

Example 7

Unfractionated novolac polymer resin and the fractionated polymers fromExamples 1-4b were combined with the surfactant, the fluorinated esterderivative denoted FC-430, provided by 3M, and the solvent, ethyllactate, to form coating solutions with the listed % solids and %solvent. 2-4 ml of the solution were dispensed at the center of 4 inchbare silicon wafers. The wafers were spun at 500 rpm for 2 secondsfollowed by a 20 second spin at 4000 rpm. The wafers were baked on a hotplate at 200° C. for 120 seconds. Observations are given below in Table3.

TABLE 3 Performance of Coating Composition Thickness Smoke Material %Solid % Solvent (Å) Observation Unfractionated 20.35 79.45  4253 Heavyfor first 20 seconds Unfractionated 36.82 62.98 14802 Heavy for first 60seconds Example 1 17.8 81.75  4323 None Example 1 31.8 67.75 14523 NoneExample 2 17.8 81.75  4261 None Example 2 31.8 67.75 13977 None Example3 15.8 83.75  3875 None Example 3 29.8 69.75 14785 None Example 4 17.881.75  4136 None Example 4 31.8 67.75 13550 None

As evidenced by the increase in glass transition temperature anddecrease in thermal weight loss reported in Table 1, fractionationresults in a novolac polymer resin with increased thermal stability. Theresults in Table 3 clearly demonstrate that using the coatingcomposition with fractionated novolac polymer resin, according to thepresent invention, no smoking or fuming is observed during the processof heating a coated substrate.

What is claimed is:
 1. A process of forming a planarized film on asubstrate comprising: applying to a surface of the substrate a polymericplanarizing solution comprising a novolac resin having a weight averagemolecular weight between about 1000 and 2300 amu, wherein the novolacresin is fractionated to remove the molecules with molecular weightbelow about 350 amu to less than about 22% of a total molecular weightdistribution and further has a polydispersity of less than 1.4 and asurfactant selected from the group consisting of a non-fluorinatedhydrocarbon, a fluorinated hydrocarbon and combinations thereof to forma coated substrate; and heating said coated substrate to form aplanarized film on the surface.
 2. The process of claim 1, wherein thesolution further comprises an organic solvent.
 3. The process of claim2, wherein the organic solvent comprises at least one of ethyl lactate,ethyl acetate, propyl acetate, and butyl acetate.
 4. The process ofclaim 1, wherein said novolac resin has a fraction with molecular weightbelow about 350 amu that is less than about 15% of the total.
 5. Theprocess of claim 1, wherein said novolac resin is a phenolic resin. 6.The process of claim 1, wherein said novolac resin has a polydispersityof less than about 1.2.
 7. The process of claim 1, wherein thesurfactant is an ester derivative of a fluorinated hydrocarbon.
 8. Theprocess of claim 1, wherein applying to a surface a polymericplanarizing solution further comprises spin-coating the solution ontothe surface.
 9. The process of claim 1, wherein heating the coatedsubstrate comprises heating the substrate by placing it on a hot plate.10. The process of claim 9, wherein the coated substrate is placed onthe hot plate for about 0.5 to 5 minutes.
 11. The process of claim 9,wherein the coated substrate is placed on the hot plate at a temperaturerange of between 50° C. and 300° C.
 12. The process of claim 11, whereinthe temperature range is between 100° C. and 200° C.
 13. The process ofclaim 1, wherein the substrate comprises silicon.
 14. The process ofclaim 13, wherein the substrate comprises a circuit pattern on thesurface of the substrate.
 15. An integrated circuit formed by theprocess of claim
 14. 16. A microelectronic device formed by the processof claim
 1. 17. A planarized film formed by the process of claim 1.